Create dependabot.yml

From https://discourse.julialang.org/t/psa-use-dependabot-to-update-github-actions-automatically/96001

Author: abhro

.github/dependabot.yml

@@ -0,0 +1,7 @@
+# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/" # Location of package manifests
+    schedule:
+      interval: "weekly"